<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>my css4</title>
    <style>
        html {
            font-size: 12pt;
            color: darkgray;
        }
        .c1 {
            font-size: 12px;
        }
        .c2 {
            font-size: 20pt;
        }
        .c4 {
            font-size: 2em;   /*当前大小的2倍*/
        }
        .c5 {
            font-size: 2rem;   /*根元素大小的2倍*/
        }
        .c6 {
            height: 50px;
            /*background: rgb(100, 100, 100);*/
            /*background: #c0a1b2;*/
            /*background: #9ac;  !*#99aacc*!*/
            background: darkcyan;
        }
    </style>
</head>
<body>
<div class="c6"></div>
<div class="c1">Hello div1</div>
<div class="c2">
    <div class="c4">Hello div2 div1</div>
    <div class="c5">Hello div2 div2</div>
</div>
<div class="c3">Hello div3</div>
</body>
</html>